Microanatomic distribution of hematopoietic stem/progenitor cells in endosteal, subendosteal, and central regions within the bone marrow cavity. (A) The relative distribution of transplanted Lin⁻c-kit⁺ cells to endosteal (< 3 cells from endosteum; zone 1), subendosteal (4-14 cells from endosteum; zone 2), and central (> 14 cells from endosteum; zone 3) in irradiated and nonirradiated hosts in compact bone is depicted. Each circle depicts the average values from all sections from one experiment (60-150 sections for compact bones and a similar number from trabecular bones per experiment). In compact bones (α4^+/+ recipients: ●, and Sl/Sl^d recipients: ), the relative frequency of cells seeding close to the endosteal surface is increased in irradiated hosts, at the expense of cell homing to central regions. This relative preference for seeding to endosteal regions in irradiated hosts was not observed when donor cells were treated with the inhibitor of G_i protein signals, pertussis toxin (○). (B) In trabecular bone, irradiation did not significantly affect the relative distribution of transplanted cells to endosteal versus nonendosteal regions.